Allergic asthma is thought to result from Th2-polarized immune responses to otherwise apparently innocuous environmental proteins. Why specific proteins drive such maladaptive immune responses in susceptible hosts has been unclear. Our lab and others have suggested that direct, (dys)functional interactions between such proteins and the innate immune system are central to the molecular basis of allergenicity. In the case of Der p 2, a major house dust mite allergen, our data indicate that it tends to be a target of adaptive immune responses because it has intrinsic adjuvant activity due to its molecular mimicry of MD-2, the LPS-binding subunit of the TLR4 complex. Der p 2 can reconstitute and augment TLR4 signaling in the absence and presence of MD-2, respectively. In addition, Der p 2 drives TLR4-dependent, MD- 2-independent Th2-polarized inflammation in the airways of mice. Recent data suggest that the relevant locus of TLR4 signaling important for driving Th2 inflammation in the airway involves TLR4 signaling by airway epithelial cells (AECs), which regulate the development of aeroallergic responses via the production of chemokines and cytokines that recruit and regulate the function of lung dendritic cells. MD-2-dependent TLR4 signaling involves activation of both Mal/MyD88 and TRIF/TRAM pathways of signal transduction. The molecular mechanisms underlying Der p 2-dependent TLR4 signaling remain to be defined. Based on preliminary data, we hypothesize that: (a) like MD-2, Der p 2 drives TLR4 signaling via both pathways; but (b) the allergenicity of Der p 2 is depends primarily on TRIF/TRAM signaling. The proposed experiments will define the signaling pathways activated by Der p 2-dependent TLR4 stimulation in genetically tractable cell lines; determine the Th2-polarizing cellular responses (and underlying signaling pathways) driven by Der p 2-driven TLR4 signaling in primary AECs; and define the TLR4-driven signaling pathways essential to the in vivo allergenicity of Der p 2 using genetic mouse models. The long-term goal of this proposal is to define the signaling pathways activated by Der p 2 and their biological consequences for aeroallergy, in order to devise new preventive and/or therapeutic strategies for allergic asthma.